I was quite amazed at the sound the simple $50 Guanzo preamp buffer produced via the ME550...; so who can say how much better a more esoteric preamp might sound. The Russian tubes got me, I must say, for the price to quality ratio they are just brilliant...

Distortion is a byproduct of gain. So you'd expect a unity gain buffer to sound good. The only thing that could beat it would be a "passive preamp" (basically a wire in a box.)

The buffer can do some impedance matching but this tread is way to long to know what you have on both sides of the buffer amp

Have you done any blind testing? It's hard because you need some way to flip a switch and bypass the buffer amp and not change the volume in the process. And of course you need an assistant to flip a coin and then hit the bypass or not. (the switch needs to be silent too, so yo can't hear if the assistant changed the setup.)

But there is no need to assume price determines quality in a small tube buffer amp. The parts are dirt cheap, A $9 tube and a few cents for passive parts is all you need UNLESS you need balanced outputs then you are into $100+ (each) for a decent transformer.

I just bought what turned out to be a pretty good microphone preamp for $29. Proof that you don't have to spend a fortune. This one even has regulated DC heater and a better mechanical design (without an exposed glass tube) You don't need one of these but as an example see the link:ART-Tube-Studio-Mic-Preamp

I've recently gotten into building studio gear and of course the signal paths are all balanced, I have a goal to learn how to build my own transformers.